Camptothecin (“CPT”) is a cytotoxic alkaloid first isolated and characterized by Wall and his coworkers (J. Am. Chem. Soc. 88, 3888, 1966) from leaves and barks of Camptotheca accuminata (Nyssaceae), a plant native to China. The primary cellular target for CPT is topoisomerase I (topo I), an enzyme involved in the relaxation of supercoiled chromosomal DNA during DNA replication by transient single-strand cleavage, unwinding, and reannealing of a DNA double helix. CPT binds at the interface of the covalent binary topo I-DNA complex to form stable ternary complex, which is prevents the reannealing or re-ligation, and consequently leads to replication-mediated double-strand breakage and DNA damage. Because CPT inhibition can lead to cell death during the S-phase of the cell cycle, CPT has been a focus of extensive studies in anticancer drug development (Nature Review/Cancer, October 2006 Vol. 6, pp 789-802; Bioorg. Med. Chem., 2004, 12, pp 1585-1604).
The native CPT has a pentacyclic structure consisting of a fused ring system of quinoline rings (rings A and B), a pyrrolidine ring (ring C), an alpha-pyridone ring (ring D), and a six-membered lactone ring (ring E). CPT has only one asymmetric center at the 20-position and displays dextro-rotation due to the S-configuration of a tertiary hydroxyl group. At pH 7 or above, the lactone ring is hydrolyzed to give the carboxylate derivative, a hydrolysis process facilitated by the hydrogen bonding interaction between the 20(S)-OH and carbonyl groups in the E-ring. The instability of the E-ring, which is exacerbated at physiological conditions where the carboxylate derivative has preferential (150-fold higher) binding to human serum albumin, is a major limitation in the clinical applications of CPT as an anticancer chemotherapeutic agent. The carboxylate derivative of CPT is not only biologically inactive but also clinically very toxic. Another deficiency of CPT as a drug molecule is its poor water solubility. The native CPT is not soluble in water or in other aqueous vehicles that are suitable for parenteral administration. For reasons discussed above, it is not feasible to develop CPT to its water-soluble carboxylate derivative. Clearly, from the above discussion, improving physiological or in-vivo stability and water-solubility of the lactone ring of the CPT is the focus of the medicinal chemistry efforts in CPT-based anticancer drug development (Bioorg. Med. Chem., 2004,12, pp 1585-1604; Chem. Rev., 2009, 109(1), pp 213-235).
It is believed that the hydrolysis of the E-ring lactone is facilitated by the hydrogen bonding interaction between the 20(S)-hydroxyl group and the neighboring carbonyl group. Previous experiments have shown that replacing the 20(S)-hydroxyl group with other groups, e.g. Methyl, or protecting the 20(S)-hydroxyl group with functional groups, e.g. ester, could result in a stable E-ring lactone at physiological conditions. However, the 20(S)-hydroxyl group is essential for the pharmacological activity of camptothecin.
Such 20(S)-hydroxyl group protection would make the drugs difficult to exert anticancer activity in the human body.
The pro-drug strategy is a good approach to introduce ionizable functional groups to improve the water-solubility of the resulting pro-drug molecule. In such a case, the pro-drug approach can convert a water-insoluble CPT into a water-soluble CPT-pro-drug. Since the water-soluble CPT-pro-drug would be rapidly distributed through the body within a short period of time after being administered into the blood stream, the CPT-pro-drug would exist at a very low concentration at the time of degradation, preventing CPT from precipitation in blood stream. Obviously the pro-drug approach has the potential to bring in lactone stability, water-solubility, and convenient drug administration to facilitate the CPT anticancer drug development.
There have been reports to prepare pro-drugs of CPT and CPT-based compounds, mostly by esterification of the 20(S)-hydroxyl group to introduce various protecting functional groups, including lipophilic and ionizable functional groups (Chem. Rev., 2009, 109(1), pp 213-235). Conversion of pro-drug esters to native CPT is mediated by a group of enzymes called esterases that are present in the blood of many animals, including humans. The weakness of the pro-drug esters is that the ester linkage is not very stable at the physiological condition in the human body, and is too easy to be broken by esterase, resulting in clinically unsatisfactory results for the CPT pro-drug esters. (Chem. Rev., 2009, 109(1), pp 213-235). The CPT 20(S)-O-Phosphate or phosphonate monoesters have also been prepared to increase water-solubility and lactone in vivo stability. However, as shown in experimental results, the 20(S)-O-phosphate or phosphonate esters could not be converted to CPT at the physiological conditions, precluding their usage as pro-drugs of CPT (Organic Lett., 2004, 6(3), pp. 321-324). The 20(S)-O-Phosphate or phosphonate monoester derivative of CPT does not have anti-cancer activities in itself.
Therefore, it is still desirable to discover a CPT pro-drug which has acceptable water solubility and acceptable enzymatic activity at the physiological conditions to liberate or promote the active component(s) or feature(s) of CPT.